6 replies to this topic

[Fr1ki]

I am designing an orbital railgun simulator in MS Excel that should be able to propel a 20kg projectile to about 1% SoL.  This would give 250 kT of TnT equivalent ( about 1TJ ) at ground zero.
I have already completed a simulator for the railgun and only need help with the ballistics of the projectile.

 

What I have : 
Final Velocity ( +- 300 000 m/s ) 
Projectile Shape ( rod like ; much like the discarding sabot rounds that tanks use )
Projectile weight ( 20kg )

Projectile density ( tungsten carbide )
Air density ( assuming incompressible flow )
Total distance that projectile has to travel ( 2000 km )
Railgun simulator ( Can achieve any exit velocity )

What i need : 
Exit velocity needed of the projectile from the barrel to achieve 1% SoL at ground zero.

 

As far as I understand, the flow regime will be turbulent all the way, and this is were I am stuck.  I do not have a value for the drag coefficient of my projectile, but I did find a few values, but they are not for a rod-like shape with a sharp point.

I think I only need a C_d value for my equation then ill be fine and a equation for its contact area.

I can attach the railgun simulator if anyone would want it but it is not needed to solve the problem, it merely supplies the initial velocity. 

Thank you for your time!

 

[Fr1ki]

After much debate with my friends, I have decided to add the equation that will be using in case I might have been unclear.

 

m*dv/dt = Fg - Fb - Fd

 

where m = mass

dv/dt = acc

Fg = Force due to gravity

Fb = buoyency  force

Fd = drag force

 

Hope this is more clear.

[Giuseppe Lombardi]

Hi,

sorry. but what do you mean for 1% Sol? and for 1TJ?

 

I wish to help you.

Giuseppe

[Fr1ki]

1% SoL -> 1% Speed of light ( about 3 000 000 m/s ) 

1 TJ ->   1 Terrajoule of kinect energy

 

 

Hope this helps

Edited by Fr1ki, 17 June 2013 - 07:40 AM.

[Art Montemayor]

What role does a rail gun play in the realm of Chemical Engineering?

 

Do we have any members in our Forums that could possibly contribute experience or knowledge to a rail gun?

[thorium90]

I can understand Art's concern. This is clearly a physics problem.

Someone with knowledge dealing with projectile motion with air resistance would be able to show you the steps. Nevertheless, I would like to point out that the problem can be taken in a simple way by taking air resistance to be proportional to velocity, or in a much more complicated way, taking into account the varying atmospheric density, temperature, etc.

The simple way yields ODEs, while the complicated way yields many PDEs to solve numercially.

 

Also, projectile motion with air resistance is a basic topic in physics taught to all undergrad physics majors. Or, if you worked in an artilliery unit...

http://en.wikipedia....of_a_projectile

 

Also, wikipedia has shared the basics to calculating drag force.

http://en.wikipedia....rag_coefficient

 

You claimed to have a spreadsheet that simulates the ballistics, perhaps you can share.

 

 

What i need : 
Exit velocity needed of the projectile from the barrel to achieve 1% SoL at ground zero.

 

I would also like to point out that the above statement does not make sense. Exit velocity of Earth is a constant, which is 11.2Km/s

http://en.wikipedia....Escape_velocity

If you were referring to the projectile velocity from the barrel, why are you stating 1%SOL? Your preliminary calculations do not make sense, perhaps your spreadsheet will clarify what you have typed.

Edited by thorium90, 17 June 2013 - 10:17 AM.

[Fr1ki]

@Art :
I am sorry if I have been unclear, but the railgun merely supplies the initial velocity.  I have received help from numerous people and university lecturers on this subject and is not at all a problem here.  I will attach it anyway, but please keep in mind, it only supplies the initial velocity.  This is an extra credit project done with extra effort ( if that makes sense )
 
@thorium90 :
After much reading, I know now that I will not be able to do this problem with compressible flow and thus not take into account variable pressure and temperature. Also it would be far too much effort to determine the Cd of a projectile of a specific design.  I will assume Cd to be 0.05, close to that of a "raindrop" shape and a jet fighter.
Projectile trajectory is of no importance to me as I will be firing it from a 90 degree angle.  If you meant the compensation needed to actually hit my target from that distance, I have a undergrad electronic engineer doing that for extra credit in one of his subjects and we will incorporate these two.

I also did not claim to have in my possession a ballistics simulator, that is what I am building now.
Regarding the statement I made : I require the final velocity to be 1% SoL which seems to be the terminal velocity as it enters the atmosphere as I will not take variable density into account.  This simplifies my problem a lot as I only need the velocity as it enters the atmosphere.  The first 1993km's that it should travel are in space, and i'll assume no deceleration in space.
 
All the questions really helped me to figure out what I should do.  It shouldn't take me more than a day to complete this thing.  I will post it as soon as I can for all to see and comment on.
It will be a small simulator.  Where the impact energy ( in TnT equivalent ) will be required along with few other things and return the strength of the railgun required to achieve this.
 
Thank you, the replies have been very fast and helpful.

Please ask if there are any more uncertainties.

Attached Files

•  Railgun.xls   67.5KB   5 downloads